No-bake foundry sand is foundry sand mixed with a self setting binder that fixes the sand in position without the necessity of baking. The most widely used no-bake binder is one which, in the presence of a catalyst, reacts to coat individual foundry sand grains with a continuous film coating that hardens on curing to produce a rigid castable sand mold without the application of heat or pressure. Typical organic no-bake binders are furan resins, phenolic urethanes and oil urethanes, while a typical inorganic binder is sodium silicate. See Haine, "Updating No-Bake Molding Systems," FOUNDRY M & T, February 1975 at page 66.
Molten metal is then poured into the no-bake sand mold to produce a casting. In casting, some of the no-bake rigid coatings are burned off from the sand grains directly exposed to the molten metal, but approximately ninety percent of the sand is fixed by encasement in the bonding film and is thus rendered useless as foundry sand for reuse unless the rigid no-bake film coating is removed from the sand. Typical means for removing coatings from foundry sand have been by heat or abrasion. Heat or thermal reclamation, is slow and expensive, while abrasion is not very effective for removing no-bake coatings because the rigid coatings are very smooth and scratch resistant.
Existing foundry sand reclaimers utilize abrasive methods for removing soft non-carbonaceous coatings such as clay, bentonite and silica from sand. These types of binding materials form soft coatings which can be readily removed by "rubbing" abrasive methods, see U.S. Pat. Nos. 4,025,419 to Muschoot, 2,707,214 to Horth and 2,478,461 to Connolly. However, such sand reclaimers depending on abrasive methods are not effective for removing no-bake coatings from foundry sand, since the abrasive action is not entirely effective for completely removing the rigid coatings.
The "air scrubber" is another type of foundry sand reclaimer. Heretofore known air scrubbers utilize "blast tubes," "gun barrels" and/or "nozzles" to accelerate sand by means of compressed air to very high velocities to break and wear off burned clay coatings; see for example, U.S. Pat. Nos. 2,119,886 and 2,119,887 to Myers. Such air scrubber reclaimers are not energy efficient, in that a lot of power is required to accelerate the sand. Moreover, the sand is accelerated to high velocities, which is not desirable for recovering no-bake foundry sand since much of the sand is thereby disintegrated during reclamation and rendered unsuitable for further use.
These sand reclaimers are also based on the "unit" design concept. The "unit" design concept consists of a single device into which used sand is charged and from which reclaimed sand is discharged. Such reclaimers suffer from the drawback that sand quality deteriorates with transporting and other handling of the sand from the reclaimer to the point of use, therefore, the reclaimed sand product is not the same at the point of use as it was at the reclaimer "unit" discharge.
In addition, heretofore known air classifiers for separating materials are only suitable for separating lighter particles from heavier denser particles. Such air classifiers utilize countercurrent air flow to float and "waft" unwanted lighter particles from a decending mixture; see, for example, U.S. Pat. No. 1,650,727 to Stebbins, U.S. Pat. No. 3,312,342 to Brown, and U.S. Pat. No. 2,203,959 to Hammack. Such air classifiers are not suitable for use in classifying foundry sand where reclaimed sand grains of a certain AFS grain fineness number and screen analysis are desired.